Flash memory with nanocrystalline silicon film floating gate

ABSTRACT

A memory is described which has memory cells that store data using hot electron injection. The data is erased through electron tunneling. The memory cells are described as floating gate transistors wherein the floating gate is fabricated using a conductive layer of nanocrystalline silicon particles. Each nanocrystalline silicon particle has a diameter of about 10 Å to 100 Å. The nanocrystalline silicon particles are in contact such that a charge stored on the floating gate is shared between the particles. The floating gate has a reduced electron affinity to allow for data erase operations using lower voltages.

TECHNICAL FIELD OF THE INVENTION

The present invention relates generally to memory circuits and inparticular the present invention relates to floating gate memory cells.

BACKGROUND OF THE INVENTION

Non-volatile memory such as electrically programmable read-only memory(EPROM) and electrically-erasable programmable read-only memory (EEPROM)are extensively used for storing data in computer systems. EPROM andEEPROM comprise a large number of memory cells having electricallyisolated gates, referred to as floating gates. Data is stored in thememory cells in the form of charge on the floating gates. Charge istransported to or removed from the floating gates by program and eraseoperations, respectively.

Another type of non-volatile memory is flash memory. Flash memory is aderivative of EPROM and EEPROM. Although flash memory shares manycharacteristics with EPROM and EEPROM, the current generation of flashmemory differs in that erase operations are done in blocks.

A typical flash memory comprises a memory array which includes a largenumber of memory cells arranged in row and column fashion. Each of thememory cells include a floating gate field-effect transistor capable ofholding a charge. The cells are usually grouped into blocks. Each of thecells within a block can be electrically programmed in a random basis bycharging the floating gate. The charge can be removed from the floatinggate by a block erase operation. The data in a cell is determined by thepresence or absence of the charge in the floating gate.

Flash memories have the potential of replacing hard storage disk drivesin computer systems. The advantages would be replacing a complex anddelicate mechanical system with a rugged and easily portable smallsolid-state non-volatile memory system. There is also the possibilitiesthat given their very high potential densities that given more speed ofoperation particularity in the erase operation that flash memories mightbe used to replace DRAMs. Flash memories might then have the ability tofill all memory needs in future computer systems.

One flash memory is described in S. Tiwari et al., "Volatile andNon-volatile Memories in Silicon with Nano-Crystal Storage," Abstr. ofIEEE Int. Electron Device Meeting, pp. 521-524 (1995), which usesconfined nano-crystal particles in a floating gate memory cell. Theindividual nano-crystals are not in electrical contact with each other,and therefore cannot share a common charge. As referred to in the art,the memory has a thin gate oxide and uses a tunnel-tunnel process forwriting and reading data. A memory designed to use a tunnel-tunnelprocess typically has a gate oxide thickness of about 15-20 Å which canbe degraded over time resulting in a defective memory.

For the reasons stated above, and for other reasons stated below whichwill become apparent to those skilled in the art upon reading andunderstanding the present specification, there is a need in the art fora fast flash memory having a floating gate memory cell which in whichthe floating gate has a reduced electron affinity, can share a commoncharge, or does not use a tunnel-tunnel process.

SUMMARY OF THE INVENTION

The above mentioned problems with memory devices and other problems areaddressed by the present invention and which will be understood byreading and studying the following specification. A memory cell isdescribed which uses a film of nanocrystalline silicon as a floatinggate.

In particular, the present invention describes an integrated circuitmemory cell comprising a transistor having a source, drain, gate and afloating gate, the floating gate comprising a film of nanocrystallinesilicon particles. The nanocrystalline silicon particles are in contactto form a conductive film.

In another embodiment, an integrated circuit field effect transistor isdescribed. The transistor comprises a conductive channel separatingsource and drain regions, a floating gate comprising a conductive filmof nanocrystalline silicon particles. The floating gate is locatedadjacent the channel and separated therefrom by a first insulatinglayer. A gate is provided adjacent the floating gate and separatedtherefrom by a second insulating layer.

In yet another embodiment, a memory device is described which has anarray of memory cells. Each of the memory cells comprise a sourceregion, a drain region, a conductive channel separating the source anddrain regions, and a floating gate comprising a conductive film ofnanocrystalline silicon particles. The floating gate is located adjacentthe channel and separated therefrom by a first insulating layer. A gateis also located adjacent the floating gate and separated therefrom by asecond insulating layer. The memory further comprises addressingcircuitry for addressing the array of memory cells, and controlcircuitry for controlling read and write operations of the memorydevice.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional view of a prior art memory cell;

FIG. 2 is the memory cell of FIG. 1 during programming;

FIG. 3 is a cross-section of a memory cell incorporating a film ofnanocrystalline silicon as a floating gate;

FIG. 4 is a graph of barrier height versus tunneling distance; and

FIG. 5 is a simplified block diagram of a typical flash memoryincorporating the present invention.

DETAILED DESCRIPTION OF THE INVENTION

In the following detailed description of the preferred embodiments,reference is made to the accompanying drawings which form a part hereof,and in which is shown by way of illustration specific preferredembodiments in which the inventions may be practiced. These embodimentsare described in sufficient detail to enable those skilled in the art topractice the invention, and it is to be understood that otherembodiments may be utilized and that logical, mechanical and electricalchanges may be made without departing from the spirit and scope of thepresent inventions. The following detailed description is, therefore,not to be taken in a limiting sense, and the scope of the presentinventions is defined only by the appended claims.

FIG. 1 is a cross-sectional view of a typical memory cell, such as aused in a flash memory. Memory cell 100 comprises a region of a source102 and a region of a drain 104. Source 102 and drain 104 areconstructed from an N+ type of high impurity concentration which areformed in a P-type semiconductor substrate 106 of low impurityconcentration. Source 102 and drain 104 are separated by a predeterminedspace of a channel region 108. Memory 100 further includes a floatinggate 110 formed by a first polysilicon (poly) layer, and a control gate114 formed by a second poly layer. Floating gate 110 is isolated fromcontrol gate 114 by an interpoly dielectric layer 112 and from channelregion 108 by a thin oxide layer 116 approximately 100 angstroms (Å)thick.

FIG. 2 is the memory cell of FIG. 1 during a programming operation.During programming, a positive programming voltage of about 12 volts isapplied to control gate 114. This positive programming voltage attractselectrons 120 from P-type substrate 106 and causes them to accumulate atthe surface of channel region 108. A voltage on drain 104 is increasedto about 6 volts, and the source 102 is connected to ground. As thedrain-to-source voltage increases, electrons 120 flow from source 102 todrain 104 via channel region 108. As electrons 120 travel toward drain104, they acquire substantially large kinetic energy and are referred toas hot electrons.

The voltages at control gate 114 and drain 104 creates an electric fieldin oxide layer 116, this electric field attracts the hot electrons andaccelerates them toward the floating gate 110. At this point, floatinggate 110 begins to trap and accumulate the hot electrons and starts acharging process. Gradually, as the charge on the floating gateincreases, the electric field in oxide layer 116 decreases andeventually loses it capability of attracting any more of the hotelectrons to floating gate 110. At this point, floating gate 110 isfully charged. The negative charge from the hot electrons collected inthe floating gate 110 raises the cell's threshold voltage (Vt) above alogic 1 voltage. When a voltage on control gate 114 is brought to alogic 1 during a read operation, the cell will barely turn on. Senseamplifiers are used in the memory to detect and amplify the state of thememory cell during a read operation. Thus, data is read from a memorycell based upon its "on" characteristics.

Electrons are removed from the floating gate to erase the memory cell.Many memories, including flash memories, use Fowler-Nordheim (FN)tunneling to erase a memory cell. The erase procedure is accomplished byelectrically floating the drain, grounding the source, and applying ahigh negative voltage (-12 volts) to the control gate. This creates anelectric field across the gate oxide and forces electrons off of thefloating gate which then tunnel through the gate oxide. For a generaldescription of how a flash memory having floating gate memory cellsoperates see B. Dipert et al., "Flash Memory Goes Mainstream," IEEESpectrum, pp. 48-52 (Oct. 1993), and incorporated herein by reference.

One of the difficulties with flash memories has been the erase operationusing Fowler-Nordheim tunneling. The erase operation requires highvoltages, and is relatively slow. Further, an erratic over erase can beinduced as a result of the very high erase voltages used. These veryhigh erase voltages are a fundamental problem arising from the highelectron affinity of bulk silicon or large grain polysilicon particlesused as the floating gate. This creates a very high tunneling barrier.Even with high negative voltages applied to the gate, a large tunnelingdistance is experienced with a very low tunneling probability forelectrons attempting to leave the floating gate. This results in longerase times since the net flux of electrons leaving the gate is low.Thus, the tunneling current discharging the gate is low. In addition,other phenomena result as a consequence of this very high negativevoltage. Hole injection into the oxide is experienced which can resultin erratic over erase, damage to the gate oxide itself, and theintroduction of trapping states.

The solution to these problems is to use a floating gate having a lowerelectron affinity for electrons. Thus, a lower barrier is provided forelectrons to escape over, or through by tunneling. Lower barriersrequire lower voltages and result in smaller tunneling distances for theelectrons during the erase operation. This results in much faster erasetimes and much less damage. The possibility of a secondary problemoccurring in the gate oxide are also reduced, such as electron traps andhole injection.

The present invention describes the use of nanocrystalline silicon filmsas a floating gate in flash memories rather than the large bulky andthick polysilicon normally used. The nanocrystalline silicon films formin silicon rich oxide after silicon implantation into the oxide andappropriate anneal conditions. The silicon crystals can be made in avariety of sizes with a uniform distribution in particle sizes byappropriate anneal conditions. Although the particles may not be formedin a uniform sphere, they can be described as having a general diameterof approximately 10 Å to 100 Å. They can also be formed by chemicalvapor deposition, by rapid thermal anneal of amorphous silicon layers orby other known techniques.

The primary advantage of a nanocrystalline film floating gate is thatthese nanoscale particles have a larger bandgap than bulk silicon due toconfinement in the small particles. A quantum mechanical effect isexperienced which results in a different band structure and widening ofthe bandgap. A wider bandgap results in a lower barrier for theelectrons at the surface of the silicon particle and a much largertunneling probability when negative potentials are applied during thewrite operation. The barrier of course can not be made too small orelectrons might be thermally excited over the barrier at high operatingtemperatures and the stored charge leak off of the floating gate in longtime periods at high temperature. Normally this is not a problem inflash memories, the barrier is so high as to result in extremely longretention times, far in excess of any requirement. The problem is thatthe barrier is too high and results in FN tunneling discharge timeswhich are too long and require voltages which are too high. Reducing thebarrier between the silicon gate particles and the gate oxide serves tosignificantly improve flash memories. These nanocrystalline siliconfilms are quite conductive even when intrinsic and can be made moreconductive by the appropriate doping as is done with larger grainpolysilicon films currently used as the gate structure in flashmemories. These floating gate films, however, need not be veryconductive since they are not used elsewhere for wiring and need only beconductive enough to allow for a redistribution of carriers in thefloating gate structure itself.

FIG. 3 is a cross-sectional view of a transistor 300 of the presentinvention. Transistor 300 comprises a region of a source 302 and aregion of a drain 304. Source 302 and drain 304 are constructed from anN+ type of high impurity concentration which are formed in a P-typesemiconductor substrate 306 of low impurity concentration. Source 302and drain 304 are separated by a predetermined space of a channel region308. Transistor 300 farther includes a floating gate 310 formed as asilicon nanocrystalline film. A control gate 314 is formed by apolysilicon layer. Floating gate 310 is isolated from control gate 314by an interpoly dielectric layer 312 and from channel region 308 by athin gate oxide layer 316. The floating gate silicon nanocrystallinefilm is comprised of nanocrystalline particles which can be embedded ineither dielectric layer 312 or 316. These particles have a diameter inthe size range of approximately 10 Å to 100 Å and are in a uniform sizedistribution for a particular set of processing conditions. Theparticles are in contact with each other and form as a result ofannealing the silicon rich oxide which follows silicon implantation intothe gate oxide or deposition of silicon and appropriate annealconditions to grow nanocrystalline particles of silicon which then forma film. This film is patterned using standard techniques known to thoseskilled in the art to form the floating gates. When used as a memorycell, the drain of the transistor is typically coupled to a bit line,and the control gate is coupled to a word line.

Using these silicon nanocrystals in a conductive film is distinctlydifferent than other techniques of using isolated silicon nanocrystalsto observe trapping of single electrons on these isolated crystals, asdescribed above. Here the nanocrystals are used as a conductive film toreplace the coarse grain polysilicon floating gate structure in aconventional flash memory structured with hot electron injection as thewrite mechanism and tunneling as the erase mechanism.

FIG. 4 illustrates how the smaller barrier at the surface of thefloating gate silicon storing electrons results in a shorter tunnelingdistance through a gate oxide insulating layer and consequently a muchhigher tunneling probability. Tunneling distance "do" represents thetunneling distance experienced in typical transistor having apolysilicon floating gate. Tunneling distance "dn" represents thetunneling distance experienced in a transistor having a floating gate asdescribed in FIG. 3. The tunneling probability is an exponentialfunction of the reciprocal tunneling distance and even a small reductionin the tunneling distance results in tunneling probabilities which areorders of magnitude higher and tunneling time which are orders ofmagnitudes smaller.

The result then is a memory cell with a much faster and much easiererase operation due to the lower barrier height for the electrons totunnel through in leaving the floating gate structures. Also, the thickpolysilicon floating gate structure is replaced by a much thinner filmof nanocrystals of silicon particles embedded in an oxide region. Thisresults in a much more compact device, and a memory cell with a planarstructure having less stray sidewall capacitive coupling.

FIG. 5 is a simplified block diagram of a typical system having a flashmemory incorporating the present invention. Memory 200 comprises amemory array 202 having memory cells. A row decoder 204 and a columndecoder 206 are designed to decode and select addresses provided onaddress lines 208 to access appropriate memory cells. Command andcontrol circuitry 210 is designed to control the operation of memory 200in response to incoming command and control signals from a processor201, including a write enable signal 212 (WE*), control lines 216 anddata communication lines 218. Furthermore, a voltage control switch 214is provided to apply appropriate voltages to the memory cells duringprogramming operation. It will be appreciated by those skilled in theart that the memory of FIG. 5 has been simplified for the purpose ofillustrating the present invention and is not intended to be a completedescription of a flash memory.

Conclusion

A memory device is described which has fast read, write and eraseoperations. The memory uses memory cell floating gate transistors with afloating fabricated of a conductive layer of nano-size silicon crystals.Although the floating gate is conductive, it need only be conductiveenough to allow for a redistribution of carriers in the floating gatestructure itself. The memory cell has a lower electron affinity thanconventional memory cells having a floating gate fabricated frompolysilicon.

Although specific embodiments have been illustrated and describedherein, it will be appreciated by those of ordinary skill in the artthat any arrangement which is calculated to achieve the same purpose maybe substituted for the specific embodiment shown. This application isintended to cover any adaptations or variations of the presentinvention. Therefore, it is manifestly intended that this invention belimited only by the claims and the equivalents thereof.

What is claimed is:
 1. An integrated circuit meemory cell comprising:atransistor having a source, drain, control gate and a floating gate, thefloating gate comprising a film of nanocrystalline silicon particlesembedded in an insulating layer, the nanocrystalline silicon particlesbeing in contact to form a conductive film, wherein the film ofnanocrystalline silicon comprises a plurality of nanoscale siliconparticles each have a general diameter in the range of about 10 Å to 100Å.
 2. An integrated circuit memory cell comprising:a transistor having asource, drain, control gate and a floating gate, the floating gatecomprising a conductive film of nanocrystalline silicon particlesembedded in an insulating layer.
 3. An integrated circuit field effecttransistor comprising:a source region; a drain region; a conductivechannel separating the source and drain regions; a floating gatecomprising a conductive film of nanocrystalline silicon particles, thefloating gate located adjacent the channel and separated therefrom by afirst insulating layer; and a control gate located adjacent the floatinggate and separated therefrom by a second insulating layer, wherein thefilm of nanocrystalline silicon comprises a plurality of nanoscalesilicon particles each having a general diameter in the range of about10 Å to 100 Å; wherein the first and second insulating layers arecomprised of oxide and the floating gate is embedded in either the firstor second insulating layer.
 4. A memory device comprising:an array ofmemory cells, each o the memory cells comprising: a source region, adrain region coupled to a bit line, a conductive channel separating thesource and drain regions, a floating gate comprising a conductive filmof nanocrystalline silicon particles, the floating gate located adjacentthe channel and embedded in a first insulating layer wherein thenanocrystalline silicon particles each have a general diameter in therange of about 10 Å to 100 Å, and a control gete located adjacent thefloating gate and separated therefrom by a second insulating layer, thecontrol gate is coupled to a word line; addressing circuitry foraddressing the array of memory cells; and control circuitry forcontrolling read and write operations of the memory device.
 5. Thememory device of claim 4, wherein the memory device is a flash memory.6. A memoory device comprising:an array of memory cells, each of thememory cells comprising: a source region, a drain region coupled to abit line, a conductive channel separating the source and drain regions,a floating gate comprising a conductive film of nanocrystalline siliconparticles embedded in a first layer of insulating material, wherein thenanocrystallin silicon particles each have a general diameter in therange of about 10 Å to 100 Å, and a control gate located adjacent thefloating gate and separated therefrom, the control gate is coupled to aword line; addressing circuitry for addressing the array of memorycells; and control circuitry for controlling read and write operationsof the memory device.
 7. A non-volatile cell comprising:a transistorhaving a source, drain, control gate and a floating gate, the floatinggate comprising a film of nanocrystalline silicon particles embedded inan insulating layer and in contact to form a conductive film, whereinthe film of nanocrystalline silicon comprises a plurality of nanoscalesilicon particles each have a general diameter in the range of about 10Å to 100 Å such that the floating gate has a low electron affinity toallow for low voltage erase operations.